1. Field of the Invention
The present invention relates to a communication method and a transmission power control method in digital radio communication, which are suitable for application to a radio communication system using the TDMA scheme, such as a radio telephone system, and relates to mobile stations using such a communication method.
2. Description of the Related Art
Heretofore, the TDMA (Time Division Multiple Access) scheme has been put to practical use as a communication scheme applied to a radio telephone system or the like. In this scheme, one frame is formed of a plurality of time slots. By using an arbitrary time slot in one frame, communication is conducted. In the TDMA scheme, by using one transmission channel, multiple communication up to the number of time slots forming one frame can be conducted at maximum.
In a communication system using this TDMA scheme as well, it is necessary to properly control transmission power from each station in the same way as other communication schemes, in order to improve the frequency utilization efficiency and minimize the interference on communication of other stations. As transmission power control processing in a conventional communication system using the TDMA scheme, only the so-called closed-loop transmission power control for setting the transmission power at each mobile station on the basis of control effected from, for example, a base station was conducted.
However, the closed-loop transmission power control which was conducted in conventional communication of the TDMA scheme uses a control loop which is comparatively slow in response. Its control range is also narrow. The conventional processing is insufficient in all of frequency, precision, and control range of transmission power control. It cannot be said that interference on other communication is suppressed to a level of the lowest limit.
Heretofore, the CDMA (Code Division Multiple Access) scheme has been developed as a communication scheme for conducting transmission power control of comparatively high precision. In the case of this CDMA scheme, however, it was basically premised on the assumption that a mobile station continuously received signals from a base station. It was difficult to apply the CDMA scheme as it was to the TDMA scheme for conducting burst transmission and reception.
An object of the present invention is to make it possible to conduct transmission power control favorably in the case where communication is conducted by using the TDMA scheme.
In the case where communication is conducted between a base station and a mobile station by using a TDMA scheme, a communication method of the present invention includes the steps, in a mobile station, of: estimating quality of a received signal by taking a time slot as unit, from a received signal in a time slot taken as unit; in response to the received signal quality estimated by taking a time slot as the unit being larger than predetermined transmission power, conducting transmission processing in a transmission time slot interval, with transmission power smaller in value than the predetermined transmission power by a difference value between the estimated received signal quality and the predetermined transmission power; and in response to the received signal quality estimated by taking a time slot as the unit being smaller than predetermined transmission power, conducting transmission processing in a transmission time slot interval, with transmission power larger in value than the predetermined transmission power by a difference value between the predetermined transmission power and the estimated received signal quality.
According to this communication method, transmission power in each time slot interval can be controlled properly on the basis of the actual received signal quality.
In the case where two-way communication is conducted between a first communication station and a second communication station by using a TDMA scheme, a transmission power control method of the present invention includes, as transmission power control in the first communication station, the steps of: receiving a signal transmitted from the second communication station, and estimating quality of a received signal by taking a time slot as unit; in response to the received signal quality estimated by taking a time slot as the unit being larger than predetermined transmission power, conducting processing of transmission to the second communication station in a predetermined time slot interval, with transmission power smaller in value than the predetermined transmission power by a difference value between the estimated received signal quality and the predetermined transmission power; and in response to the received signal quality estimated by taking a time slot as the unit being smaller than predetermined transmission power, conducting processing of transmission to the second communication station in a predetermined time slot interval, with transmission power larger in value than the predetermined transmission power by a difference value between the predetermined transmission power and the estimated received signal quality.
According to this transmission power control method, the transmission power in the first communication station can be controlled properly on the basis of actual reception quality of a signal transmitted from the second communication station.
Furthermore, a mobile station of the present invention includes: an estimation means for estimating quality of a received signal by taking a time slot as unit, from a signal received by a reception means by taking a time slot as unit; a comparison means for comparing the received signal quality estimated by the estimation means by taking a time slot as the unit with a first reference value; and a control means, responsive to the received signal quality being larger than the first reference value as a result of comparison conducted in the comparison means, for producing a control signal so as to reduce transmission power by a difference value between the received signal quality and the first reference value, and responsive to the received signal quality being smaller than the first reference value as a result of comparison conducted in the comparison means, for producing a control signal so as to increase transmission power by a difference value between the first reference value and the received signal quality, the transmission power in the transmission means being subjected to variable control according to the control signals.
According to this mobile station, transmission power can be controlled properly on the basis of actual received signal quality of a signal transmitted from the base station.